1. Field Of The Invention
The present invention relates to an apparatus and method to control the capacity of a reciprocating type pump generally and preferably or most usefully a compressor. By controlling the volumetric capacity of the pump, the mass through-put is controlled. In the case of a Rankine cycle machine, the current invention together with standard suction or discharge control, would provide a direct means to control the heat pumping capacity of the cycle without compromising efficiency.
2. Description Of The Related Art
According to the 1996 ASHRAE Handbook "Systems and Equipment Handbook (SI)", page 34.8, capacity control may be obtained by one or more of the following:
(1) controlling suction pressure by throttling; PA1 (2) controlling discharge pressure; PA1 (3) returning discharge gas to suction; PA1 (4) adding re-expansion volume; PA1 (5) changing the stroke; PA1 (6) opening a cylinder discharge port to suction while closing the port to discharge manifold; PA1 (7) changing compressor speed; PA1 (8) closing off cylinder inlet, and PA1 (9) holding the suction valve open. PA1 Continuous adjustment to load PA1 Full-load efficiency unaffected by the control PA1 No loss in efficiency at part load PA1 Reduction of starting torque PA1 No reduction in compressor reliability PA1 No reduction in compressor operating range PA1 No increase in compressor vibration and sound level at part load
The ASHRAE handbook states that the most commonly used methods are the opening of the suction valves by some external force, gas bypassing within the compressor, and gas bypassing outside the compressor. Most of these techniques seriously compromise the compressor efficiency. Changing compressor speed does not directly compromise efficiency but does have the practical problem of exciting various structural resonances as the speed changes. Stroke variation is another technique that avoids efficiency penalties provided that the top dead center (TDC) clearance volume is minimized. One means of achieving stroke control is to connect the piston directly to a linear motor plunger. In this case the piston position is free, that is, it is not fixed by the kinematic geometry of the machine. Capacity control is achieved directly since the linear motor plunger amplitude is controllable. This configuration is generally referred to as the "Linear Compressor". The major difficulty with the linear compressor is the control of the approach clearance between the piston and the valve plate at TDC. In order to achieve high efficiencies, the clearance needs to be as small as possible. Even a momentary loss of control could result in the piston colliding with the valve plate resulting in catastrophic damage. Linear compressor design generally compromises efficiency by increasing the TDC clearance so as to minimize collision problems.
An alternative method of capacity control employed by linear compressors is to vary the dead space at TDC. This technique is referred to as adding re-expansion volume. In this case efficiency is directly compromised by introducing severe irreversibilities associated with hysteresis losses.
According to the 1996 ASHRAE Handbook "Systems and Equipment Handbook (SI)", page 34.8, an ideal capacity control system would have the following operating characteristics:
The object of the present invention is to meet all of the ideal characteristics in a simple direct manner.